/**
 * @author Deepkolos / https://github.com/deepkolos
 */

export class WorkerPool {

    constructor(pool = 4) {
        this.pool = pool;
        this.queue = [];
        this.workers = [];
        this.workersResolve = [];
        this.workerStatus = 0;
    }

    _initWorker(workerId) {
        if (!this.workers[workerId]) {
            const worker = this.workerCreator();
            worker.addEventListener('message', this._onMessage.bind(this, workerId));
            this.workers[workerId] = worker;
        }
    }

    _getIdleWorker() {
        for (let i = 0; i < this.pool; i++)
            if (!(this.workerStatus & (1 << i))) return i;
        return -1;
    }

    _onMessage(workerId, msg) {
        const resolve = this.workersResolve[workerId];
        resolve && resolve(msg);
        if (this.queue.length) {
            const {resolve, msg, transfer} = this.queue.shift();
            this.workersResolve[workerId] = resolve;
            this.workers[workerId].postMessage(msg, transfer);
        } else {
            this.workerStatus ^= 1 << workerId;
        }
    }

    setWorkerCreator(workerCreator) {
        this.workerCreator = workerCreator;
    }

    setWorkerLimit(pool) {
        this.pool = pool;
    }

    postMessage(msg, transfer) {
        return new Promise((resolve) => {
            const workerId = this._getIdleWorker();
            if (workerId !== -1) {
                this._initWorker(workerId);
                this.workerStatus |= 1 << workerId;
                this.workersResolve[workerId] = resolve;
                this.workers[workerId].postMessage(msg, transfer);
            } else {
                this.queue.push({resolve, msg, transfer});
            }
        });
    }

    destroy() {

        this.workers.forEach((worker) => worker.terminate());
        this.workersResolve.length = 0;
        this.workers.length = 0;
        this.queue.length = 0;
        this.workerStatus = 0;

    }

}
